Absolutely agree with this.
I was the SEN governor for a primary school for over five years, including a stint as acting Chair of Governors.
Statistical information such as the number of SEN children is absolutely not confidential, unless disclosing the number in a certain class could identify a specific child.
As PP have said, schools provide the statistics differently, but the overarching principle is that parents are entitled to know the demographics of the pupil population, including the number of SEN children.
Our school had around 35% SEN pupils and that was considered to be extremely high. We had a specialist unit attached to the mainstream - a Communication and Interaction Centre. This operated as a special school, with the pupils not following the National Curriculum. However, it provided the opportunity for some access to mainstream classes for pupils who were able to manage limited time (with support obviously).
Because of the C&I Centre, our school had a reputation for being outstanding for SEN, and we had a lot of pupils arrive after they'd been in a PRU because they couldn't manage at other mainstream schools. The proximity and expertise within the C&I Centre, and the fact that these pupils spent time in the mainstream meant that even our mainstream teachers were extremely proficient in supporting SEN.
However, lack of funding and resources in the current education system means that there's not an infinite pot to dip into, and some of our most gifted pupils weren't stretched enough. We did occasionally have parents remove their child from the school because they felt they weren't being pushed hard enough and would achieve more elsewhere. Teachers aren't magicians and with the current trend of forcing SEN children into a mainstream environment - regardless of whether it's the best place for them - there is always going to be some fallout. Certainly in our area there are schools which are recognised for having high achieving pupils but perhaps aren't as good with SEN, and other schools who are outstanding at identifying and supporting SEN, but don't get such high scores on SATs etc.